cpp-lazy-stream/streams/filter.hpp

#pragma once

namespace stream {
  template <typename T>
  struct filter_t {
    template <typename F>
    explicit filter_t(F&& f) : pred(f) {}
    std::function<bool(const T&)> pred;
  };
}

template <typename T, bool B>
stream::detail::stream_base<T, true> operator|(stream::detail::stream_base<T, B> const&s, stream::filter_t<T>&& f) {
  return s.filter(f.pred);
}
template <typename T, bool B>
stream::detail::stream_base<T, true> operator|(stream::detail::stream_base<T, B> &&s, stream::filter_t<T>&& f) {
  return std::move(s).filter(f.pred);
}

namespace stream {
  namespace detail {
    template <typename T, bool B>
    class filter_stream : public stream_impl<T> {
    public:
      class iterator : public iterator_impl<T> {
      public:
        typedef iterator_impl<T> super;
        iterator(std::function<bool(T const&)> f, ::stream::iterator<T>&& impl)
        : pred_(f), impl_(std::forward<::stream::iterator<T>>(impl)), next_(impl_) {
          advance();
        }
        ~iterator() {}
        T operator*() override { return mem_; }
        
        super& operator++() override {
          ++impl_;
          advance();
          return *this;
        }
        
        DELEGATE_ITERATOR_IMPL_BASE(impl_)
      private:
        void advance() {
          while (!pred_(mem_ = *impl_)) {
            ++impl_;
          }
        }
        
        std::function<bool(T const&)> pred_;
        T mem_; // To avoid re-calcs, we store this
        ::stream::iterator<T> impl_, next_;
      };
      
      template <typename F>
      filter_stream(F&& func, stream_base<T, B> const& sb) : pred_(func), source_(sb) {}
      template <typename F>
      filter_stream(F&& func, stream_base<T, B> && sb) : pred_(func), source_(std::forward<stream_base<T, B>>(sb)) { }
      ~filter_stream() override {}
      
      ::stream::iterator<T> begin() override { return new iterator{pred_, source_.begin()};}
      ::stream::iterator<T> end() override { return new iterator{pred_, source_.end()}; }
    private:
      std::function<bool(T const&)> pred_;
      stream_base<T, B> source_;
    };
    
    template <typename T, bool Own>
    template <typename F>
    auto stream_base<T, Own>::filter(F&& pred) const& -> stream_base<T, true>  {
      using impl_t = filter_stream<T, false>;
      return stream_base<T, true>{new impl_t{pred, *this}};
    }
    
    template <typename T, bool Own>
    template <typename F>
    auto stream_base<T, Own>::filter(F&& pred) && -> stream_base<T, true> {
      using impl_t = filter_stream<T, Own>;
      return stream_base<T, true>{new impl_t{pred, std::forward<stream_base<T, Own>>(*this)}};
    }
  }
}